It is generally desirable for electric motors, including spoked magnet motors, to achieve the best torque-per-amp operating points. At such operating points, the motor uses the minimum current necessary to drive the output torque. This may be accomplished by synchronizing the stator and rotor, but doing so requires knowing the position of the rotor as accurately as possible. One way to directly determine the position of the rotor is to use a Hall effect sensor, encoder, or resolver in the motor.
Another way to determine the position of the rotor is to use a sensorless algorithm based on back electromotive force (EMF), inductance, and resistance to predict the position of the rotor. Sensorless algorithms are relatively accurate, but under some conditions can be sufficiently inaccurate so as to significantly lower motor efficiency. Angle errors of three degrees (3°) or less may have little or no significant effect, but angle errors of six degrees (6°) or seven (7°) degrees may lower motor efficiency by approximately one-half percent (0.5%) and increase motor phase current by approximately one percent (1%). Given that the difference between normal and high performance motors may be as little as three percent (3%), even relatively small angle errors can significantly lower motor efficiency.
This background discussion is intended to provide information related to the present invention which is not necessarily prior art.